1. The vast majority of bone mineralization in girls occurs by the middle of the second decade. 2. Premature bone demineralization occurs in women with hypothalamic dysfunction manifest as amenorrhea and oligomenorrhea, associated with athletics, dancing, and eating disorders. 3. In young women with amenorrhea associated with weight loss, BMD loss will be occurring soon after the amenorrhea develops. Treatment to prevent BMD loss or promote BMD accretion should begin soon, probably within 6 months after amenorrhea occurs. 4. Women who recover from anorexia nervosa at a young age (< 15 years of age) can have normal total body BMD, but regional (lumbar spine and femoral neck) BMD may remain low. The longer the anorexia nervosa persists, the less likely it is that the BMD will return to normal. Girls and women with anorexia nervosa need to be rehabilitated early in the disease to maximize BMD accretion. 5. Conjugated estrogen, in doses that improve bone mineralization in postmenopausal women and in combination with medroxyprogesterone, has not been shown to improve BMD in young women with hypothalamic amenorrhea. The role of orally administered medroxyprogesterone at a dose of 10 mg per day, 10 days per month, in improving BMD in teenage girls with hypothalamic amenorrhea or oligomenorrhea remains to be established. 6. Treatment with OCP may have a beneficial effect on BMD in young women with hypothalamic amenorrhea, but this has not been established in a double-masked, randomized, controlled trial. Doing a double-masked trial using OCP will be difficult because estrogen-deficient subjects treated with OCP will be likely to have menstrual bleeding, whereas those treated with placebo will not. In addition, the risk of pregnancy in a sexually active subject, who does not know whether she is receiving OCP, is too great for some subjects. 7. Osteoporosis is a major cause of morbidity and death. Peak bone mass is a major determinant of the risk of osteoporosis, and the second decade is the critical period of peak bone mass acquisition; thus providers of health care for adolescents need to understand the factors that affect bone mineralization during this period, and advise patients accordingly.